Conventional methods to achieve desired agronomic phenotypes such as increased yield, disease prevention, disease resistance, and improved abiotic stress tolerance have utilized mostly selective breeding, grafting, transgenic and agrochemical approaches.
Bioactive Priming Polypeptides Involved in Plant Defense Responses
Plants possess an immune system that detects and protects against microbes that can cause disease. Antimicrobial peptides (AMPs) in plants are often the first line of defense against invading pathogens and are involved in the initiation of defense responses that can impart innate immunity to a plant. Many AMPs are generically active against various kinds of infectious agents. They are generally classified as antibacterial, anti-fungal, anti-viral and/or anti-parasitic.
The resistance of given plant species against certain pathogenic organisms that can contact a plant surface and colonize it, is based on highly specialized recognition systems for molecules produced only by certain microbes (for example, specific bacterial or fungal strains). Plants sense potential microbial invaders by using pattern-recognition receptors (PRRs) to recognize the pathogen-associated molecular patterns (PAMPs) associated with them.
Flagellin/Flagellin-Associated Polypeptides
Flagellins and flagellin-associated polypeptides derived from those flagellins have been reported primarily to have functional roles in innate immune responses in plants. These polypeptides are derived from highly conserved domains of eubacterial flagellin. Flagellin is the main building block of the bacterial flagellum. The flagellin protein subunit building up the filament of bacterial flagellum can act as a potent elicitor in cells to mount defense-related responses in various plant species.
“Flagellin” is a globular protein that arranges itself in a hollow cylinder to form the filament in a bacterial flagellum. Flagellin is the principal substituent of bacterial flagellum, and is present in flagellated bacteria. Plants can perceive, combat infection and mount defense signaling against bacterial microbes through the recognition of conserved epitopes, such as the stretch of 22 amino acids (Flg22) located in the N-terminus of a full length flagellin coding sequence. The elicitor activity of Flg22 polypeptide is attributed to this conserved domain within the N-terminus of the flagellin protein (Felix et al., 1999). Plants can perceive bacterial flagellin through a pattern recognition receptor (PRR) at the plant's cell surface known as flagellin sensitive receptor, which is a leucine-rich repeat receptor kinase located in the plasma membrane and available at the plant cell surface. In plants, the best-characterized PRR is FLAGELLIN SENSING 2 (FLS2), which is highly conserved in both monocot and dicot plants.
In Arabidopsis, the innate immune response to Flg22 involves a host recognition protein complex that contains the FLS2 leucine rich repeat (LRR) receptor kinase (Gómez-Gómez L. and Boller T., “FLS2: An LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis,” Molecular Cell 5: 1003-1011, 2000). In Arabidopsis thaliana, FLS2 is a PRR that determines flagellin perception and is specific for the binding of the flagellin-associated polypeptide(s). For example, the binding of Flg22 to the outer plant FLS2 membrane-bound receptor triggers a signaling cascade that is involved in the innate immune response that induces the plant to mount a highly specific signaling-associated cascade that is involved in the activation of pattern-triggered immunity (Chinchilla et al., “The Arabidopsis receptor kinase FLS2 binds Flg22 and determines the specificity of flagellin perception,” Plant Cell 18: 465-476, 2006). Thus, the binding of Flg22 to the Arabidopsis FLS2 membrane-bound receptor promotes the first step of activation in which the binding elicits an activation cascade for defense responses in the plant. The Flg22-FLS2 interaction can also lead to the production of reactive oxygen species (ROS) that contribute to the induction of an oxidative burst, cellular medium alkalinization, downstream induction of pathogen-responsive genes and defense-related responses which then can impart disease resistance to a plant (Felix G. et al., “Plants have a sensitive perception system for the most conserved domain of bacterial flagellin,” The Plant Journal 18: 265-276, 1999, Gómez-Gómez L. and Boller T., “FLS2: An LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis,” Molecular Cell 5: 1003-1011, 2000, Meindi et al., “The bacterial elicitor flagellin activates its receptor in tomato cells according to the address-message concept,” The Plant Cell 12: 1783-1794, 2000). In tomato, high affinity binding of Flg22 to a FLS receptor was observed using both intact cells as well as to microsomal membrane preparations. In this study, the binding of Flg22 to the FLS2 receptor(s) at the plasma membrane surface was nonreversible under physiological conditions, which reflects an uptake process of the Flg22 elicitor with import into the tomato cells (Meindi et al., “The bacterial elicitor flagellin activates its receptor in tomato cells according to the address-message concept,” The Plant Cell 12: 1783-1794, 2000). Recognition of Flg22 by FLS2 triggers both local and systemic plant immune responses. The Flg22-bound, activated FLS2 receptor complex is internalized into plant cells by endocytosis and moves systemically throughout the plant (Jelenska et al., “Flagellin peptide flg22 gains access to long-distance trafficking in Arabidopsis via its receptor, FLS2,” Journal of Experimental Botany 68: 1769-1783, 2017), which may contribute towards systemic Flg22 immune responses.
Flagellin receptor perception mediation involving Flg22 is highly conserved across divergent plant taxa (Taki et al., “Analysis of flagellin perception mediated by flg22 receptor OsFLS2 in rice,” Molecular Plant Microbe Interactions 21: 1635-1642, 2008). Submicromolar concentrations of synthetic polypeptides comprising between 15-22 or 28 amino acids from conserved domains of a flagellin protein, act as elicitors to initiate defense responses in a variety of plant species.
Generation of transgenic plants has been used to confirm the flagellin-specific PAMPs that bind to the flagellin-specific PRRs. Ectopic expression of FLS2 in Arabidopsis plants showed a direct correlation between the flagellin responses and FLS2 expression levels, which indicate that FLS2 is involved in the recognition of flagellin (a signal of bacterial presence) and leads to the activation of defense responses in plants (Gómez-Gómez L. and Boller T., “FLS2: An LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis,” Molecular Cell 5: 1003-1011, 2000). Transgenic plants expressing the flagellin binding receptor have shown efficacy against certain pathogens. Flagellin binding to FLS2 was involved in the initiation of expression of specific MAP kinase transcription factors that function downstream of the flagellin receptor FLS2. Mutant plants (fls2) lacking in the FLS2 receptor are insensitive to Flg22 (Gómez-Gómez L. and Boller T., “FLS2: An LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis,” Molecular Cell 5: 1003-1011, 2000), and impaired in Flg22 binding to the FLS2 receptor. Mutant plants (fls2) also exhibited enhanced susceptibility to infection and disease when treated with pathogenic bacteria (Zipfel et al., “Bacterial disease resistance in Arabidopsis through flagellin perception,” Nature 428: 764-767, 2004).
Traditionally, methods to improve disease resistance have capitalized on these and other such findings and have taken a transgenic approach. Transgenic plants and seeds transformed with a Flagellin-Sensing (FLS) receptor protein (WO2016007606A2 incorporated herein by reference in its entirety) or with transcription factors involved in downstream signaling of FLS (WO2002072782A2 incorporated herein by reference in its entirety) have produced plants that confer disease resistance to certain pathogenic microorganisms. In another example, transgenic plants expressing Flagellin-Sensing (FLS3) receptor also have exhibited enhanced resistance to disease compared to non-transgenic plants not expressing the FLS3 receptor (WO2016007606A2 incorporated herein by reference in its entirety).
Plant Defensins/Thionins
Plant defensins are also characterized as anti-microbial peptides (AMPs). Plant defensins contain several conserved cysteinyl residues that form disulphide bridges and contribute to their structural stability. Defensins are among the best characterized cysteine-rich AMPs in plants. Members of the defensin family have four disulfide bridges that fold into a globular structure. This highly conserved structure bestows highly specialized roles in protecting plants against microbial pathogenic organisms (Nawrot et al., “Plant antimicrobial peptides,” Folia Microbiology 59: 181-196, 2014).
Thionins are cystine-rich plant AMPs classified in the defensin family and typically comprise 45-48 amino acid residues, in which 6-8 of these amino acids are cysteine that form 3-4 disulfide bonds in higher plants. Thionins have been found to be present in both monocot and dicot plants and their expression can be induced by infection with various microbes (Tam et. al., “Antimicrobial peptides from plants,” Pharmaceuticals 8: 711-757, 2015). Particular amino acids of thionins such as Lys1 and Tyr13, which are highly conserved, have been found to be vital to the functional toxicity of these AMPs.
Harpin and Harpin-Like (HpaG-Like)
Similar to the flagellins or the flagellin-associated polypeptides, harpins comprise a group of bacterial-derived elicitors that are derived from larger precursor proteins. Harpins are critical for the elicitation of a hypersensitive response (HR) when infiltrated into the intercellular space or apoplast of plant cells (Kim et al., “Mutational analysis of Xanthomonas harpin HpaG identifies a key functional region that elicits the hypersensitive response in nonhost plants,” Journal of Bacteriology 186: 6239-6247, 2004). Application of the distant harpin-like (HpaG-like) bioactive priming polypeptide(s) to a plant provides an alternative conduit to protect a plant from disease and insect pressure. Harpins utilize a type III secretion system that enable the transport of proteins across the lipid bilayers that makeup the plant plasma cell membrane. The binding of harpins to the surface of the plasma cell membrane can trigger an innate immune response that resembles those triggered by pathogen-associated molecular patterns (PAMPs) and are known to activate PAMP-triggered immunity (Engelhardt et al., “Separable roles of the Pseudomonas syringae pv. phaseolicola accessory protein HrpZ1 in ion-conducting pore formation and activation of plant immunity,” The Plant Journal 57: 706-717, 2009). Mutational analysis of a harpin-like HpaG derived polypeptide showed that the 12 amino acid residues between Leu-39 and Leu50 of the original 133 amino acid harpin elicitor precursor protein was critical to the elicitation of a hypersensitive (HR) and subsequent innate immune responses in tobacco (Kim et al., “Mutational analysis of Xanthomonas harpin HpaG identifies a key functional region that elicits the hypersensitive response in nonhost plants,” Journal of Bacteriology 186: 6239-6247, 2004). This indicates that a specific amino acid region of harpins (similar to the other AMPs) is responsible for the elicitation responses. Harpins, such as HpaG-like can be used to enhance resistance to not only plant pathogens but also to insects (Choi et al., “Harpins, multifunctional proteins secreted by gram-negative plant pathogenic bacteria,” Molecular Plant Microbe Interactions 26: 1115-1122, 2013). Harpin has been used to induce disease resistance in plants and protect plants from colonization and feeding by insect phloem-feeding insects, such as aphids (Zhang et al., “Harpin-induced expression and transgenic overexpression of phloem protein gene At.PP2A1 in Arabidopsis repress phloem feeding of the green peach aphid Myzus persicae,” BMC Plant Biology 11: 1-11, 2011).
Elongation Factor Tu (EF-Tu)
Elongation factor Tu is an abundant protein found in bacteria and acts as a pathogen-associated molecular pattern (PAMP) to initiate signaling cascades that are involved in plant disease resistance and plant innate immunity to microbial pathogenic organisms. Interestingly, some EF-Tu polypeptides are also found to exist in plants. The first 18 amino acid residues of the N-terminus of EF-Tu from Escherichia coli, termed elf18, is known to be a potent inducer of PAMP-triggered immune responses in plants (Zipfel et al., “Perception of the bacterial PAMP EF-Tu by the Receptor EFR restricts Agrobacterium-mediated transformation,” Cell 125: 749-760, 2006). Polypeptides derived from E. coli EF-Tu are perceived by the plant cell-surface localized receptor EF-Tu receptor (EFR) (Zipfel et al., 2006). EF-Tu binding and activation of EFR follow a similar mode of action compared to that of the Flg peptide-FLS2 receptor complex (Mbengue et al., “Clathrin-dependent endocytosis is required for immunity mediated by pattern recognition receptor kinases,” Proc Natl Acad Sci U.S.A. 113: 11034-9, 2016).
Growth Altering Bioactive Priming Polypeptides
Phytosulfokines (PSKα)
Phytosulfokines (PSK) belong to a group of sulfated plant polypeptides that are encoded by precursor genes that are ubiquitously present and highly conserved in higher plants (Sauter M., “Phytosulfokine peptide signaling,” Journal of Experimental Biology 66: 1-9, 2015). PSK genes are encoded by small gene families that are present in both monocots and dicots and encode a PSK polypeptide(s) that can be active as either a pentapeptide or a C-terminally truncated tetrapeptide (Lorbiecke R, Sauter M, “Comparative analysis of PSK peptide growth factor precursor homologs,” Plant Science 163: 348-357, 2002).
The phytosulfokine protein is targeted to the secretory pathway in plants by a conserved signal polypeptide (Lorbiecke R, Sauter M, “Comparative analysis of PSK peptide growth factor precursor homologs,” Plant Science 163: 348-357, 2002). Processing of the phytosulfokine precursor protein involves sulfonylation by a tyrosylprotein sulfotransferase within the plant secretory pathway, specifically the trans-Golgi followed by secretion and proteolytic cleavage in the apoplast in order to produce PSK (Sauter M., “Phytosulfokine peptide signaling,” Journal of Experimental Biology 66: 1-9, 2015). After PSK is processed from the larger precursor polypeptide, the polypeptide undergoes tyrosine sulphation (Ryan et al., “Polypeptide hormones,” The Plant Cell Supplement, S251-S264, 2002). The secreted polypeptide is then perceived at the cell surface by a membrane-bound receptor kinase of the leucine-rich repeat family (Sauter M., “Phytosulfokine peptide signaling,” Journal of Experimental Biology 66: 1-9, 2015 where PSK can then bind to the specialized PSK receptor (for example, PSK1 from Arabidopsis) which has a leucine-rich repeat region located on the plant plasma membrane surface. Specific binding of PSK was detected in plasma membrane fractions from cell suspension cultures derived from rice and maize and the binding to the receptor was shown to initiate and stimulate cell proliferation (Matsubayashi et al., “Phytosulfokine-α, a sulfated pentapeptide, stimulates the proliferation of rice cells by means of specific high- and low-affinity binding sites,” Proceedings National Academy of Science USA 94:13357-13362, 1997).
Phytosulfokines (PSK) serve as sulfated growth factors with biostimulant activities and are involved in the control of the development of root and shoot apical meristems, growth regulation and reproductive processes. PSKs have also been reported to initiate cell proliferation, differentiation of quiescent tissues and are involved in the formation and stimulation and differentiation of tracheary elements (Matsubayashi et al., “The endogenous sulfated pentapeptide phytosulfokine-α stimulates tracheary element differentiation of isolated mesophyll cells of zinnia, Plant Physiology 120: 1043-1048, 1999). PSK signaling has also been reported to be involved in the regulation of root and hypocotyl elongation that occurs in Arabidopsis seedlings (Kutschmar et al., “PSK-α promotes root growth in Arabidopsis,” New Phytologist 181: 820-831, 2009).
Root Hair Promoting Polypeptide (RHPP)
Root hair promoting polypeptide (RHPP) is a 12 amino acid fragment derived from soybean Kunitz trypsin inhibitor (KTI) protein, which was detected from soybean meal that was subjected to degradation using an alkaline protease from Bacillus circulans HA12 (Matsumiya Y. and Kubo M. “Soybean and Nutrition, Chapter 11: Soybean Peptide: Novel plant growth promoting peptide from soybean,” Agricultural and Biological Sciences, Sheny H. E. (editor), pgs. 215-230, 2011). When applied to soybean roots, RHPP was shown to accumulate in the roots and promote root growth through the stimulation of cell division and root hair differentiation in Brassica. 